Combined co2 capture and conversion method and system

ABSTRACT

Systems and methods are described that utilize mixed conductive membranes in conjunction with solid oxide electrolysis in a single unit operation. The mixed conductive membranes of the systems are high-flux electrochemical separation membranes that can selectively and efficiently capture CO 2  from a source gas, e.g., a flue gas or fuel gas, by transporting the CO 2  across the membrane in the form of carbonate ions followed by oxidation of the carbonate-ions at the second side of the membrane to reform CO 2  in a capture stream. The solid oxide electrolysis can be immediately downstream of the capture system to convert the CO 2  into CO and H 2 O into H 2 , for example thereby forming syngas thereby capturing CO 2  and converting it to a more useful form.

CROSS REFERENCE TO RELATED APPLICATION

This application claims filing benefit of U.S. Provisional Patent Application Ser. No. 61/959,159 having a filing date of Aug. 16, 2013 entitled “Combined CO₂ Capture and Conversion Reactor,” which is incorporated herein.

BACKGROUND

Climate change is one of the greatest challenges in human history. Manmade CO₂ produced through the use of fossil fuels is thought to be the major source for climate change. To mitigate manmade climate change without significantly affecting the growth of economy and our life style, the stabilization of atmospheric CO₂ concentration is perceived to be the most realistic near-term solution. The mainstream technical approach to achieving stable atmospheric CO₂ concentration is to curb the emission of CO₂ from the existing and new large-scale fossil-fueled power plants by capturing CO₂ at point-sources and geologically storing it.

Three industrial combustion processing points have been examined as CO₂ removal point sources: pre-combustion, post-combustion and oxy-combustion. In pre-combustion CO₂ capture, CO₂ is removed from a CO₂—H₂ mixture produced by a water-gas-shift reaction of syngas prior to combustion. It is suited for integrated gasification combined cycle (IGCC) power plants, where coal gasification-derived syngas is the primary fuel source for power generation. In post-combustion capture, CO₂ is separated from a N₂-rich flue gas after the fossil fuel has been combusted in air, and before the combustion by-products are released into the atmosphere. Capture of CO₂ through oxy-combustion involves combusting fossil fuels in almost pure O₂ rather than air, which requires a constant supply of pure O₂ produced from air. This process results in a flue gas of almost pure CO₂ and steam. The CO₂ can then be separated relatively easily from the mixture for transportation and storage.

Current CO₂-capture/separation technologies are principally based on reversible chemical/physical sorption processes using liquid solvents and solid sorbents as a CO₂ scrubber. Typical examples of sorption materials under development include solvent-based amine and ionic liquids, and sorbent-based activated carbon and molecular sieves. Unfortunately, the cost and energy penalty to implement these scrubbing technologies into existing power plants are so high that the overall plant efficiency and the cost of electricity could be significantly impacted. For example, the parasitic loads (steam and power) required to support post-combustion CO₂ capture would decrease power generation capacity of the power plant by nearly one-third and increase levelized cost of electricity by as much as 80%. This remains the major challenge to commercial deployment of these technologies. A great deal of global research and development activities have been devoted to this area in recent years in an effort to reduce the cost and energy consumption associated with sorption-based CO₂ capture processes.

An appealing alternative to geologic storage of captured CO₂ is to have CO₂ recycled back to fuel form. The significance of this technological development is enormous: it rebalances the carbon cycle in our ecosystem and enables a sustainable energy future. With renewables as the energy input to the conversion, the making of synthetic fuels from CO₂ can be carbon-neutral or even carbon-negative. Moreover, it can reduce the costs and energy associated with CO₂ capture.

An alternative to solvent and sorbent-based CO₂ capture is membrane-based capture techniques. Membrane based CO₂ capture approach presents advantages in cost and energy consumption due to its ability to deliver high-pressure CO₂, to promote CO shifting reaction and to not use energy-intensive steam or chemical load. Typical examples of membrane materials include size-exclusion based inorganic Al₂O₃, Zeolites, carbons, SiO₂, and organic polymers. Unfortunately, these membranes have limited selectivity, and as such provide product purity. Moreover, these membranes generally exhibit poor compatibility with high-temperature. Therefore, developing alternative high-temperature membranes to capture CO₂ more selectively and efficiently for existing fossil-fueled power plants would be of great benefit.

Petroleum has been an indispensable fuel for a long period of time. However, petroleum is neither a renewable nor sustainable energy resource. With current rate of petroleum consumption, it is estimated to be depleted in less than 100 years. As such, the search for alternative energy sources to replace or reduce petroleum use is imperative to our nation's energy security as well as energy independence. Options for obtaining meaningful reductions in petroleum use include developing technologies for synthesizing liquid transportation fuels.

There are two abundant domestic sources with potential to be the precursors for producing liquid fuels: biomass and coal. Two key conversion technologies currently under active commercial development are: (1) biochemical conversion, which uses enzymes to break down starch, cellulose, or hemicellulose from biomass into sugars that can be further converted into ethanol, and (2) thermochemical conversion, which uses heat and steam to convert biomass and/or coal into syngas from which ultraclean and low-sulfur liquid fuels are synthesized through the Fisher-Tropsch process. FIG. 1 summarizes the platforms/pathways currently being developed for these conversions.

However, the use of biomass and coal to synthesize liquid fuels imposes a range of potential environmental impacts on land, water, air and human health. The targeted replacement of 15% of liquid fuel currently consumed in the transportation section (1.7-2.5 million barrels per day of gasoline equivalent) with cellulosic ethanol and coal- and biomass-based liquid fuels having near-zero life-cycle CO₂ emissions would require the annual harvesting of 550 million dry tons of biomass and an increase in coal extraction in the United States by 50% over current levels. Since life-cycle CO₂ emissions to the atmosphere from coal-based liquid fuels generation are twice those of petroleum-based fuels generation, the ability to capture and store CO₂ released during coal conversion processes becomes a key to producing competitive liquid fuels from coal with life-cycle CO₂ emissions comparable to gasoline and diesel. However, in addition to problems detailed above, geologic storage of CO₂ has yet to be commercially demonstrated on a large scale in the United States.

An attractive solution to avoid geologic storage is to convert the captured CO₂ back into fuels. Two processes developed so far for such conversion are: photocatalysis and high-temperature electrolysis. For the photocatalysis process, sunlight is utilized as the energy source to split water into hydrogen and oxygen and to reduce CO₂ using earth-abundant, robust light-absorber materials with optimal bandgaps. Parallel to that effort is the development of high-temperature and high-pressure solid oxide electrolyzers (SOEs) to convert CO₂ and H₂O into syngas with a desirable H₂/CO ratio by a controlled electrochemical reduction “co-electrolysis”.

What are needed in the art are processes and systems that can capture carbon dioxide from gas stream and convert the captured carbon dioxide to a more useful form. It would be greatly beneficial if such processes and systems can be readily retrofitted into the existing coal- and natural-gas fired power plants that currently dominate in the power industry.

SUMMARY

According to one embodiment, a method for selective capture and conversion of carbon dioxide from a gaseous stream is described. A method can include contacting a feed stream with a mixed carbonate ion conducting membrane. The mixed carbonate ion conducting membrane includes a first side and a second opposite side. More specifically, the feed stream contacts the first side, and the feed stream includes carbon dioxide. The carbon dioxide is reduced to form carbonate ion upon contact between the feed stream and the first side following which the carbonate ion migrates across the mixed carbonate ion conducting membrane to the second side. The method also includes contacting the second side of the mixed carbonate ion conducting membrane with a capture gas stream (e.g., steam or syngas). The carbonate ion is oxidized at the second side of the mixed carbonate ion conducting membrane to form carbon dioxide, and the carbon dioxide is then collected by the capture gas stream.

Immediately downstream of the mixed carbonate ion conducting membrane, the method can include contacting the capture gas stream containing the carbon dioxide and H₂O (steam) with an electrode of a solid oxide electrolysis cell. The carbon dioxide and steam of the capture gas stream are reduced at the electrode to form carbon monoxide, hydrogen and oxide ion. The oxide ion migrates across the electrolyte of the solid oxide electrolysis cell.

A carbon dioxide capture and conversion system for carrying out the method is also disclosed. For instance, a system can include a feed stream, the first side of the mixed carbonate ion conducting membrane in fluid communication with the feed stream, the capture gas stream in fluid communication with the second side of the mixed carbonate ion conducting membrane, and the solid oxide electrolysis cell immediately downstream of the mixed carbonate ion conducting membrane. The solid oxide electrolysis cell comprises a first electrode, an electrolyte, and a second electrode, the first electrode being in fluid communication with the capture gas stream.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates methods under development and use for producing liquid fuels from biomass and coal.

FIG. 2 illustrates a mixed oxide and carbonate ion conducting (MOCC) membrane-based carbon dioxide separation system and the surface reactions for the membrane.

FIG. 3 illustrates a mixed electron and carbonate ion conducting (MECC) membrane-based carbon dioxide separation system and the surface reactions for the membrane.

FIG. 4 is schematically illustrates a capture-and-conversion all-in-one CO₂ system that includes an MECC membrane-based separation process and a solid oxide electrolysis (SOE) process using syngas as the capture gas.

FIG. 5 schematically illustrates a capture-and-conversion all-in-one CO₂ system that includes an MECC membrane-based separation process and an SOE process using steam as the capture gas.

FIG. 6 schematically illustrates a capture-and-conversion all-in-one CO₂ system that includes an MOCC membrane-based separation process and an SOE process using steam as the capture gas.

FIG. 7 illustrates an Sm-doped CeO₂ (SDC) membrane and includes the reconstructed X-ray computed tomography (X-CT) three-dimensional (FIG. 7A) and two-dimensional (FIG. 7B) views, FIG. 7C provides a scanning electron microscope (SEM) image of the SDC membrane and FIG. 7D illustrates the pore size distribution of the SDC membrane.

FIG. 8 shows the uniform microstructure and elemental distributions of an SDC membrane after molten carbonate is impregnated into the porous SDC structure. FIG. 8A shows the microstructure of the membrane; also illustrated is the elemental distribution of cerium (FIG. 8B), sodium and potassium (FIG. 8C), and samarium (FIG. 8D).

FIG. 9 presents a comparison of CO₂ permeability of the MOCC membrane of FIG. 8 with other oxide-carbonate systems reported in the literature

FIG. 10 illustrates the porous structure of a silver network in an MECC membrane.

FIG. 11 illustrates the microstructure of the MECC membrane of FIG. 10 and the elemental distribution of silver (FIG. 11B), aluminum (FIG. 11C), and potassium (FIG. 11D).

FIG. 12 illustrates the CO₂ flux density of the MECC membrane of FIG. 10 compared with other metal-carbonate systems reported in the literature.

DETAILED DESCRIPTION

The following description and other modifications and variations to the present disclosure may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole and in part. Furthermore, those of ordinary skill in the art will appreciate that the following description is by way of example only, and is not intended to limit the invention.

In general, disclosed herein is a reactor system and method that combines high-temperature CO₂ membrane-based capture with high-temperature solid oxide electrolysis (SOE) cells to provide a route for capturing of CO₂ followed by conversion into syngas in a single reactor unit operation. The unified system has a great potential to be cost effective and energy efficient for the ultimate synthesis of artificial fuels from fossil fuel power plants with zero carbon emission.

Gas separation membranes encompassed herein include mixed ionic and electronic conductor membranes that can meet the requirements of cost and energy consumption for the desired uses. Different from its size-exclusion rivals, the ionic transport membranes only allow the electrochemically active species to pass through the membrane in the form of charged species (ions) under a gradient of its own electrochemical potential. The mixed conductor membranes allow two or more electrochemically active species to pass through the membranes. Thereby, the selectivity of the membrane is exclusive. Since this type of membrane also operates at elevated temperatures and high pressures, it also has an excellent compatibility with high-temperature and high-pressure streams commonly met in fossil-fuel combustion processes. One previously known type of mixed conductive membrane is the oxygen transport membrane, which has been successfully developed and demonstrated for producing pure oxygen and synthetic gas in large scale. The key feature of oxygen transport membranes is the high oxygen flux achieved by the internal fast concomitant oxide-ion and electronic transport. This leads to a much simpler, more cost-effective and energy efficient reactor design than electrically driven oxygen pumps.

The disclosed systems and methods utilize mixed conductive membranes in conjunction with solid oxide electrolysis in a single unit operation. More specifically, the mixed conductive membranes of the systems and methods are high-flux electrochemical separation membranes that can selectively and efficiently capture CO₂ from a source gas, e.g., a flue gas or fuel gas, by transporting the CO₂ across the membrane in the form of carbonate ions and oxidation of the carbonate at the second side of the membrane to reform CO₂ in a capture stream.

The thus-captured CO₂ can then be further treated in the single unit operation according to an electrolysis process to covert components of the capture stream to carbon monoxide and hydrogen (i.e., syngas) in a first product stream and oxygen in a second stream.

This capture-and-conversion all-in-one CO₂ reactor system and method has advantages in energy efficiency and system cost since it avoids cooling/reheating and depressurizing/pressurizing of the captured CO₂ stream during the conversion. More specifically, the process heat held by the high-temperature CO₂-containing capture stream can be directly utilized for the endothermic electrolysis reaction in the conversion process, and this can lead to highly beneficial cost and energy savings.

In one embodiment, the mixed carbonate ion conducting membrane can be a mixed oxide-ion and carbonate-ion conductor (MOCC). In another embodiment, the selective CO₂ capturing mixed carbonate ion conducting membrane can be a mixed electron and carbonate-ion conductor (MECC). Both of these types of mixed ion conducting membranes are CO₂-selective separation membranes and they are technically and economically more attractive than previously known electrically driven molten carbonate fuel cell based CO₂ concentrators since no external electronics are needed.

FIG. 2 illustrates a working model of an MOCC membrane. Such membranes have been described in U.S. Pat. No. 8,506,677 to Huang, which is incorporated herein by reference. As can be seen, the CO₂ transport through the MOCC membrane is taken in the form of carbonate ion, CO₃ ²⁻ that is charge-compensated by the concomitant opposite flow of oxide ion, O²⁻.

The ionization and reduction of the CO₂ takes place on a first surface of the MOCC membrane, where the feed gas interaction with the O²⁻ that is delivered via a flux driven by the gradient of electrochemical potential of oxygen.

This particular embodiment, which utilizes an MOCC membrane, can be particularly well suited for CO₂ capture from a reducing, CO₂-rich atmosphere such as a pre-combustion fuel mixture. For example, a CO₂ atmosphere containing about 30% or more, or about 40% CO₂ can be fed to the MOCC membrane at temperatures from 400-700° C. In one embodiment, the feed gas can include a CO₂ and H₂ mixture that is the product of water-gas-shift reaction such as may be utilized in a steam reforming facility for the production of hydrogen. The water-gas shift reaction (WGS) is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen:

CO_((g))+H₂O_((v))→CO_(2(g))H_(2(g))

The mixed conducting MOCC membrane can include two phases at the operating conditions. The first phase can be a solid oxide porous phase that conducts oxide ion and the second phase can be a carbonate that is molten at the system operating conditions and conducts carbonate ion. The carbonate can be positioned within the solid oxide porous substrate.

The solid oxide can be a cerium oxide system, a zirconium oxide system, mixture thereof, or the like. For example, the solid oxide can be doped with gadolinium, Sm₂O₃, etc. In one embodiment, the solid oxide system of the membrane can be a homogeneous and porous samarium-doped cerium (SDC), e.g., Ce_(0.8)Sm_(0.2)O_(1.9), a formation method for which is described in co-owned patent application Ser. No. 13/957,999, which is incorporated herein by reference. Briefly, according to this embodiment, a homogeneous porous solid oxide substrate can be formed having a predetermined porosity and pore size by control of the volume fraction of sacrificial material and sintering temperature used during the formation. The method for synthesis can include co-precipitating the oxide ceramic with a sacrificial material such as NiO, Fe₂O₃, etc., to form a composite. The composite is then sintered and the sacrificial material of the sintered composite is reduced. For instance, the composite can be sintered at a temperature of from about 1000° C. to about 1700° C. The sintered composite can be reduced with H₂ at a suitable temperature, e.g., from about 500° C. to about 1000° C. After the reduction, the resultant elemental metal of the sacrificial material can then be chemically removed by dissolving it, for instance in a suitable acid, leaving behind the porous substrate.

The functionality of the porous solid oxide substrate is multiple. First, it can provide pathways for oxide ions of the electrochemical reactions occurring at the surfaces. Second, the physical pores in the porous structure serve to contain the molten carbonate, which can be held by capillary force while in the molten phase. The solid oxide substrate also acts as a mechanical support of the membrane.

The carbonate can be an alkali metal or alkaline earth metal carbonate salt or mixture thereof that is molten at the operating conditions of the unit. By way of example, in one embodiment the carbonate can be a binary eutectic mixture of Li₂CO₃ and K₂CO₃.

During the CO₂ capture process using an MOCC membrane, the carbonate ions migrate in molten carbonate phase from high end of chemical potentials of CO₂ in the feed gas (e.g., H₂ and CO₂ containing feed gas) to the low end of CO₂ in the capture gas. The reactions at the two reactive surfaces are expressed by:

CO₂+O²⁻→CO₃ ²⁻ feed gas side

CO₃ ²⁻→CO₂+O²⁻ capture gas side

At the capture gas side, the permeated CO₂ can be collected by vacuum or flushed with steam. Thus, the capture gas can be H₂O in one embodiment, and the effluent following CO₂ capture can include pure CO₂ or a mixture of H₂O and CO₂, depending upon the specific capture gas utilized, as the oxide ion will permeate back through the membrane.

FIG. 3 illustrates another embodiment in which the mixed carbonate ion conducting membrane is a mixed electron and carbonate ion conducting (MECC) membrane. Such membranes have been described in U.S. Published Patent Application No. 2011/0168572 to Huang, which is incorporated herein by reference. As can be seen, in this embodiment, the carbonate ion is transported across the membrane in the form of CO₃ ²⁻, as with the MOCC membrane, but in this embodiment, the carbonate ion is charge-compensated by electrons.

The ionization of the CO₂ is realized in the MECC embodiment in and oxidizing atmospheres by oxygen and electrons. Thus, it can be particularly well suited for CO₂ capture from an oxidizing CO₂-rich stream such as flue gas containing CO₂ and O₂, optionally in conjunction with H₂O and N₂ as may be found in post-combustion flue gas. A feed gas can be a post-combustion process including for example, typically from about from about 10% to about 15% CO₂.

As with the MECC membrane, the membrane can include two phases at the operating conditions. One phase is a porous substrate that is solid at the operating conditions and conducts electrons. The second phase is a carbonate phase that is molten at the system operating conditions and conducts carbonate ion. As with the MOCC membrane, the carbonate can be positioned within the solid porous metal substrate.

The porous substrate may include any suitable metal such as pure nickel, pure silver, or combinations of metals. The metal substrate provides functions similar to the solid oxide substrate of the MOCC membrane, and conducts electrons rather than oxide ion. The carbonate can be an alkali metal or alkaline earth metal carbonate salt or mixture thereof as described above for the MOCC membrane.

During the CO₂ capture process using an MECC membrane, the carbonate ions migrate in the molten carbonate phase from high end of chemical potentials of CO₂ and O₂ in the feed gas stream to the low end in the capture gas. The reaction at the feed gas side reactive surface is expressed by:

CO₂+0.5O₂+2e ⁻=CO₃ ²⁻ feed gas side

The reaction(s) at the capture gas side reactive surface can vary depending upon what is utilized as the capture gas. In one embodiment, the capture gas can be water (steam). In this embodiment, the reaction at the capture gas side reactive surface is expressed by:

CO₃ ²⁻→CO₂+0.5O₂+2e ⁻ capture gas side

In this embodiment, the effluent following capture of the CO₂ can thus include CO₂, and O₂ as well as H₂O from the capture gas.

In another embodiment, the capture gas can be syngas, for instance from a coal gasification reaction. In this embodiment, the reactions at the capture gas side reactive surface are expressed by:

CO+CO₃ ²⁻→2CO₂+2e ⁻

H₂+CO₃ ²⁻→H₂O+CO₂+2e ⁻

In this embodiment, the effluent following capture of the CO₂ can include CO₂, and H₂O as well as CO and H₂ of the syngas capture gas.

Following the capture of the CO₂ according to either an MECC process or an MOCC process, the combined unit operation includes conversion of the captured CO₂ to obtain syngas via an electrolysis process utilizing a solid oxide electrolyte. This may be particularly beneficial in the MECC capture scenario, as the substantial waste heat produced from the heavy exothermic oxidation reaction can be utilized for the endothermic steam-H₂O co-electrolysis to make syngas (CO+H₂).

The ability to instantly convert the captured CO₂ into syngas without changing process conditions can be of great benefit in terms of lowering carbon dioxide formation as well as increasing energy efficiency and output of a syngas formation process.

The overall chemical reaction of the co-electrolysis process of the single unit operation can include reduction of the captured carbon dioxide to form carbon monoxide and oxygen and can in one embodiment be expressed by:

CO₂+H₂O+electricity+heat→CO+H₂+O₂,

with (CO+H₂) and O₂ being separate streams within the electrolysis cell. Since high temperature co-electrolysis can receive a high-temperature and high-pressure H₂O—CO₂ stream, it has garnered much attention in recent years as a potential energy-efficient technology to make synthetic fuels. Recent progress in high-temperature H₂O—CO₂ co-electrolysis using SOE cells has evidently demonstrated great potentials of the technology to produce syngas with high efficiency, high yield and favorable H₂/CO ratio for Fisher-Tropsch processing. The present disclosure provides improved potential to the previously known systems as through providing the co-electrolysis reactants in a readily useful form via the CO₂ capture.

As illustrated in FIG. 4, FIG. 5, and FIG. 6, the specifics of the combined unit operation can vary, depending upon the nature of the capture process. For instance, FIG. 4 illustrates a combined unit operation in which flue gas is processed to capture and convert CO₂ by use of an MECC membrane using syngas as the capture gas. As can be seen, the effluent from the CO₂ capture includes CO₂, H₂O, CO and H₂. The reaction of the syngas with the carbonate ion at the permeate side of the capture membrane forms the CO₂ and H₂O in an exothermic reaction, which releases the electrons for flow back through the MECC membrane.

In the embodiment of FIG. 5, steam is the capture gas, and as such the CO₂ and O₂ formed at the permeate side of the MECC membrane will be contained in the effluent stream that will also include the H₂O.

Regardless of the type of capture gas employed, the effluent from the CO₂ capture process, which is then immediately utilized in the solid oxide electrolysis (SOE) step, is rich in H₂O and CO₂, making it ideal for the downstream high temperature co-electrolysis syngas production.

The solid oxide electrolysis process utilizes an ionic oxygen conducting electrolyte with suitable cathode and electrode materials, as is generally known in the art. For example, the electrolyte may be an oxide ion conducting ceria-based electrolyte (eg. gadolinium-doped ceria (GDC or CGO), samarium-doped Ceria (SDC)) or zirconia-based electrolyte (eg. Yttrium stabilized zirconia (YSZ), Scandium-doped zirconia (ScSZ)), which may be the same or different from the oxide ion-conduction substrate of an MOCC membrane. The electrodes can include oxygen deficient ferrites (e.g. nickel ferrite, copper ferrite, etc.) and/or any suitable perovskite electrode material as is known in the art (e.g., lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium ferrite (LSF), lanthanum strontium cobalt oxide (LSC), lanthanum strontium manganite (LSM), etc.) in the form of particles combined with the corresponding electrolyte material. For example, an electrode can include, without limitation, LSCF/GDC composite, LSM/GDC composite, LSC/GDC composite, LSWYSZ composite, and so forth that can serve as electrodes for anode or cathode respectively.

On the capture gas side, the electrolysis of the CO₂ capture gas effluent stream can vary, depending upon the specific capture gas utilized in the CO₂ capture process. In any case, however, the overall reaction can include conversion of the CO₂ and H₂O to syngas and oxygen. For instance, as illustrated in the FIG. 4, when utilizing syngas as the CO₂ capture gas for an MECC membrane-based capture process, the reactions at the SOE electrode can include:

CO₂+2e ⁻→CO+O²⁻

H₂O+2e ⁻→H₂+O²⁻

CO+H₂O→CO₂+H₂

As illustrated in FIG. 5, when utilizing steam as the CO₂ capture gas for an MECC membrane-based capture process, the reactions at the SOE electrode can include:

CO₂+2e ⁻→CO+O²⁻

H₂O+2e ⁻→H₂+O²⁻

0.5O₂+2e ⁻→O²⁻

CO₂+H₂→CO+H₂O

FIG. 6 illustrates the combined system for an MOCC membrane-based CO₂ capture process utilizing steam as the capture gas. At the permeate side, a reversal electrochemical reaction takes place to release O²⁻ back to the feed-side through oxide-ion conducting phase and gaseous CO₂ that are immediately swept away by the steam. The reactions at the SOE electrode can include:

CO₂+2e ⁻→CO+O²⁻

H₂O+2e ⁻→H₂+O²⁻

0.5O₂+2e ⁻→O²⁻

CO₂+H₂→CO+H₂O

Different from the MECC/SOE reactor, the O₂ pumped through the solid oxide electrolysis process in an MOCC membrane-based system can beneficially be separated from the purified H₂ stream.

The overall products of the combined capture and conversion CO₂ system include two separated streams: an oxygen stream and syngas. The MOCC membrane-based system can also provide a stream of H₂. The oxygen can be fed back to the original feed stream (for instance the CO₂ depleted flue gas stream of and MECC membrane-based system) for release or can be simply released or utilized in a combustion process, and the syngas can be further processed according to known methodology as desired.

The need for CO₂ capture and conversion to mitigate global warming and climate change calls for development of cost-effective and energy efficient carbon capture technology. The newly emerged high-flux electrochemical CO₂-selective membranes offer a viable technical solution to achieving that goal. Combining the electrochemical CO₂ membranes with solid oxide electrolysis cells represents a novel design of capture and conversion systems that can realize capture of CO₂ and instant conversion into syngas in a single operation. Such an “all-in-one” CO₂ reactor presents a huge cost and efficiency benefit compared to conventional CO₂ capture and geological storage, and therefore has a great potential to be implemented into the existing and new coal- and natural gas fired power plants. The accomplishments achieved in recent years in the development of MOCC and MECC membranes along with maturing SOE technology have laid a solid foundation for realizing the commercialization of the disclosed cost-effective and energy-efficient capture and conversion all-in-one CO₂ system.

Example

Dual-phase MOCC (Sm-doped CeO₂ (SDC) and carbonate) and MECC (silver and carbonate) membranes were developed for high-flux and high-efficiency electrochemical CO₂ capture. FIG. 7 shows a reconstructed 3D microstructure of a porous SDC network including a 3D rendering (FIG. 7A), a 2D rendering (FIG. 78), an SEM image (FIG. 7C) and a graphical depiction of the pore size distribution (FIG. 7D). As can be seen, the SDC substrate had evenly distributed pores.

FIG. 8 illustrates the uniform microstructure (FIG. 8A) and elemental distributions of cerium (FIG. 8B), sodium (FIG. 8C) and samarium (FIG. 8D) of the MOCC membrane after molten carbonate was impregnated into the porous SDC substrate.

FIG. 9 graphically illustrates the CO₂ permeability of the dual-phase MOCC membrane for several different ratios of SOC oxide ion conductor (SDC) to carbonate ion conductor (MC). These values are compared in the figure to permeability for other known metal carbonate systems include [1] M. Anderson et al. (“Composite electrolyte membranes for high temperature CO₂ separation”, J. Membr. Sci., 2010, 357, 122-129) and [2] J. L. Wade, et al. (“Composite electrolyte membranes for high temperature CO₂ separation”, J. Mem. Sci., 2011, 369, 20-29). As can be seen, the MOCC membranes can provide a CO₂ flux density almost two orders of magnitude higher than the comparison systems.

FIG. 10 illustrates an exemplary microstructure of a silver porous network showing uniform pore distribution in an MECC membrane. In FIG. 11 can be seen the uniform microstructure and elemental distribution of the MECC membrane after molten carbonate is impregnated into the porous silver structure.

FIG. 12 shows the high flux of CO₂ produced from the MECC membrane as compared to a previously known membrane described by [3] Y. Zhang et al. (“Sustainable chemistry: imidazolium salts in biomass conversion and CO₂ fixation”, Energy Environ. Sci., 2010, 3, 408-417). The MECC membrane provided the highest CO₂ flux density among all metal-carbonate systems investigated.

It will be appreciated that the foregoing examples, given for purposes of illustration, are not to be construed as limiting the scope of this disclosure. Although only a few exemplary embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this disclosure. Accordingly, all such modifications are intended to be included within the scope of this disclosure which is defined in the following claims and all equivalents thereto. Further, it is recognized that many embodiments may be conceived that do not achieve all of the advantages of some embodiments, yet the absence of a particular advantage shall not be construed to necessarily mean that such an embodiment is outside the scope of the present disclosure. 

What is claimed is:
 1. A method for selective capture and conversion of carbon dioxide from a gaseous stream comprising: contacting a feed stream with a mixed carbonate ion conducting membrane, the mixed carbonate ion conducting membrane having a first side and a second opposite side, the feed stream contacting the first side, the feed stream including carbon dioxide, the carbon dioxide being reduced to form carbonate ion upon contact between the feed stream and the first side, the carbonate ion migrating across the mixed carbonate ion conducting membrane to the second side; contacting the second side of the mixed carbonate ion conducting membrane with a capture gas stream, the carbonate ion being oxidized at the second side of the mixed carbonate ion conducting membrane to form carbon dioxide, the carbon dioxide being collected by the capture gas stream; immediately downstream of the second side of the mixed carbonate ion conducting membrane, contacting the capture gas stream with an electrode of a solid oxide electrolysis cell, the capture gas stream containing the carbon dioxide and steam, the carbon dioxide and steam of the capture gas stream being reduced at the electrode to form carbon monoxide and hydrogen and oxide ion, the oxide ion migrating across an electrolyte of the solid oxide electrolysis cell.
 2. The method of claim 1, wherein the feed stream is a precombustion gaseous stream.
 3. The method of claim 1, wherein the feed stream comprises hydrogen (H₂).
 4. The method of claim 1, wherein the feed stream is a post-combustion flue gas stream.
 5. The method of claim 1, wherein the feed stream comprises oxygen (O₂) and/or nitrogen (N₂).
 6. The method of claim 1, wherein the mixed carbonate ion conducting membrane conducts oxide ion in addition to the carbonate ion.
 7. The method of claim 1, wherein the mixed carbonate ion conducting membrane conducts electrons in addition to the carbonate ion.
 8. The method of claim 1, wherein the capture gas stream comprises water.
 9. The method of claim 1, wherein the capture gas stream comprises carbon monoxide and hydrogen (H₂).
 10. The method of claim 9, wherein the capture gas stream comprises syngas.
 11. A carbon dioxide capture and conversion system comprising: a feed stream; a mixed carbonate ion conducting membrane comprising a first side and a second side, the first side of the mixed carbonate ion conducting membrane having a first side and a second opposite side, the first side being in fluid communication with the feed stream; a capture gas stream, the capture gas stream being in fluid communication with the second side of the mixed carbonate ion conducting membrane; a solid oxide electrolysis cell immediately downstream of the mixed carbonate ion conducting membrane, the solid oxide electrolysis cell comprising a first electrode, an electrolyte, and a second electrode, the first electrode being in fluid communication with the capture gas stream.
 12. The system of claim 11, wherein the mixed carbonate ion conducting membrane also conducts oxide ions.
 13. The system of claim 12, wherein the mixed carbonate ion conducting membrane comprises a doped cerium oxide.
 14. The system of claim 13, wherein the dopant comprises Sm₂O₃.
 15. The system of claim 11, wherein the mixed carbonate ion conducting membrane also conducts electrons.
 16. The system of claim 15, wherein the mixed carbonate ion conducting membrane comprises silver.
 17. The system of claim 11, wherein the mixed carbonate ion conducting membrane comprises an alkali metal or an alkaline earth metal salt, or a mixture of such salts.
 18. The system of claim 11, wherein the solid oxide electrolysis cell comprises an electrolyte that conducts oxide ions.
 19. The system of claim 11, the second electrode of the solid oxide electrolysis cell being in fluid communication of the feed stream.
 20. The system of claim 11, wherein the system is a unit operation of a syngas formation process.
 21. The system of claim 11, wherein the system is a unit operation of a steam reforming process.
 22. The system of claim 11, wherein the system is a unit operation that is downstream of a water-gas shift reaction. 